Typical buildings comprise building apertures which provide a route for exchange, ventilation, circulation and/or movement of gas through the building envelope. Such gases may comprise air or water vapour, for example. Buildings may have ventilation systems, which take in “fresh” air from outside of the building and expel “stale” air from inside the building. Fresh air may be taken into a building or stale air may be expelled from a building through one or more building apertures. Some buildings incorporate other systems and/or apparatus, such as air conditioning systems and forced air clothes dryers, which require gas exchange between the inside and outside of a building through building aperture(s).
Buildings may comprise vent apparatus, each of which is associated with one or more building apertures. Such vent apparatus (or simply “vents”) are typically in fluid communication with their one or more associated building aperture(s) to provide a means for gas exchange through the building envelope. Vents may provide a number of additional functions. For example, vents may comprise weatherproofing features to minimize the amount of moisture which flows into their associated building aperture(s). Vents may provide a more aesthetically pleasing terminus for their associated building aperture(s). Vents may also incorporate means to control the flow of gases and/or other materials through their associated building aperture(s). For example, vents may restrict the back-flow of expelled gases or other external matter through their associated building aperture(s) and into the interior of the building.
There are many vent designs known in the art. For example:                Canadian patent No. 2,062,907 (Sirjoo) discloses a vent incorporating an adjustable screw cap vent cover which extends outwardly from the external wall of a building and which is adjustable to permit air flow through the vent when the cap is open and to prevent air flow through the vent when the cap is closed; and        Canadian patent No. 2,357,531 (Myint) shows a security air vent which allows for the flow of air, but which comprises a screen having S-shaped structural members for preventing the back flow of solids or liquids into the associated building aperture.        
Vents may be installed in a variety of external building surfaces, such as the walls or the roof, for example. Vents are typically installed between the layers of a building's external surface, during construction and/or finishing. For example, vents may include one or more laterally and/or vertically extending flange(s) which are installed between an interior sheathing layer and an exterior siding layer of a building wall. The installation of vents between the interior and exterior layers of a building wall causes difficulties when the vent must be replaced (for example, when the vent is broken). Replacement of such vents requires dismantling one or more exterior layer(s) of the wall in a vicinity of the vent. Once the exterior wall layer(s) are removed from the vicinity of the vent, the damaged vent may be removed and/or replaced. After replacement of the vent, the exterior wall layer(s) must be rebuilt around the new vent. For this reason, vent replacement can be an expensive, arduous and time-consuming task.
Some vent apparatus comprise a vent cover which extends outwardly from the exterior surface of the building. Such vent covers may provide weatherproofing for the vent and its associated building aperture(s) and may also provide desirable aesthetics. Vents and vent covers are typically formed in a single unitary construction.
Vent covers are particularly susceptible to damage which may be caused, for example, by exposure to natural elements (i.e. weather and temperature), age, physical blows, etc. Because of the unitary construction of vents and vent covers, however, replacement of a damaged vent cover usually requires replacement of the entire vent apparatus, which requires dismantling and rebuilding of the building surface layer(s) as described above.
Vents and their associated building aperture(s) cause an interruption in the external surfaces of buildings. If a vent is not adequately sealed, moisture may intrude into or between the layers of the building surface, damaging the building surface over time and eventually resulting in the need for repair or replacement of the building surface. Moisture or other foreign material may also intrude past the vent, into the associated building aperture(s) and possibly into the building itself. For these reasons, there is a general desire to provide vents which deter inward movement of moisture and other foreign material through the vent and prevent or minimize the intrusion of moisture between building surface layers.